1. Field of Use
This invention relates to automotive transmissions. More specifically, this invention relates to a method of converting a three-speed transmission to a two-speed transmission for use specifically in drag racing.
2. Description of the Related Art
Transmissions are used in automobiles to transmit torque from the engine to the wheels and tires to move the car down the road. To do this efficiently, they use a series of gears to provide several different gear ratios. Automatic transmissions typically come in two speeds or three speeds. In certain situations, for example drag racing, it is preferable to use a two-speed transmission. In some types of drag racing, drivers do not design and build their own transmissions, but rather they use stock production transmissions from ordinary automobiles. The drivers are therefore limited in their choice of a transmission to those two-speed transmissions which have already been designed and built by the major auto manufacturers. In order to increase the number of transmission types from which a driver may choose, it is therefore desirable to have a method of converting a three-speed transmission to a two-speed transmission.
In order to transmit torque from the engine to the wheels, automatic transmissions typically use, among other things, planetary gear systems. Indeed, the Simpson gear set, which is a compound planetary gear system, is used in transmissions made by all of the major auto manufacturers. The Simpson gear set and related improvements have been the subject of many patents, including U.S. Pat. Nos. 2,518,824; 2,806,388; 2,856,794; 2,856,795; 2,865,230; 2,873,624; 2,873,625; 3,217,563; and 3,319,491; all of which are hereby expressly incorporated by reference.
One three speed transmission that utilizes a Simpson planetary gear set is the Turbo Hydramatic 350 (or TH 350) manufactured by General Motors. Due to the many variations in transmission design, the discussion herein will focus on converting a TH 350 into a two-speed transmission. To the extent possible, however, the discussion will be kept general, and it should be understood that the potential applications of the present invention are not limited to the TH 350. A thorough discussion of the structural and operational characteristics of an unmodified TH 350 transmission, and other similar transmissions, is contained in "Automatic Transmissions and Transaxles," Harper Collins Publishers, 2nd ed., 1989; which also is hereby expressly incorporated by reference.
Referring now to FIGS. 1A, 1B, 1C, and 1D a simplified schematic representation of a compound planetary gear set 10 as found in the TH 350 transmission is illustrated. The arrows illustrate the motion of the components of the planetary gear set 10 while the transmission is in first, second, and third speed. More specifically, FIGS. 1A and 1B represent the TH 350 gear set in first speed (or drive low), FIG. 1C represents the TH 350 gear set in second speed (or drive second), and FIG. 1D represents the TH 350 gear set in third speed (or drive high). The discussion herein will be limited to the operation of the transmission in neutral, first speed, second speed, and third speed. The operation of the transmission while it is in reverse, park, manual low, and manual second will not be discussed, since those speeds are not of interest to drag racers.
The compound planetary gear set 10 comprises a front planetary gear set 12 and a rear planetary gear set 32. The front gear set 12 comprises an internal ring gear 14 having teeth 16, a plurality of planet pinion gears 20 having teeth 22, and a sun gear 26 having teeth 27. The planet gears 20 are each mounted to a planet carrier assembly 24 by a plurality of shafts 18. Additionally, the teeth 16 of the ring gear 14 match the teeth 22 of the planet gears 20, which further match the teeth 27 of the sun gear 26.
The rear planetary gear set 32 similarly comprises an internal ring gear 34 having teeth 36, a plurality of planet pinion gears 40 having teeth 42, and a sun gear 46 having teeth 47. Also, the planet gears 40 are each mounted to a planet carrier assembly 44 by a shaft 38. Further, the teeth 36 of the ring gear 34 match the teeth 42 of the planet gears 40, which further match the teeth 47 of the sun gear 46. Finally, the sun gear 26 of the front gear set 12 and the sun gear 46 of the rear gear set 32 have a common shaft 28 which connects the two sun gears 26, 46.
Torque is transmitted from the engine to the wheels by connecting the automobile engine to an input shaft of the transmission (via a torque converter), and by connecting an output shaft of the transmission to the wheels (none of which elements are illustrated). None of the members of the compound planetary gear set are attached directly to an input shaft, however. Rather, a combination of clutches is used to connect the input shaft to the internal ring gear 14 of the front planetary gear system 12. On the other hand, two members of the gear set (the from carrier 24 and the rear ring gear 34) are splined to the output shaft. Hence, one or the other of these two units is always the final driving member of the compound planetary gear set 10.
Referring now also to Table 1, it is seen that the compound planetary gear system 10 can be used to form a transmission having multiple speeds. These different speeds are attained by holding stationary one or more of the ring gears 14, 34; the planet carrier assemblies 24, 44; or the sun gears 26, 46 stationary; while allowing the other members to rotate. By varying which members are held stationary and which are allowed to rotate, different input-output gear ratios are achieved, and a transmission having multiple speeds is obtained. Table 1 illustrates the status of various clutches while the transmission is in first, second, and third speeds. Two types of clutches are used to hold members of the planetary gear set stationary. The first type is a one-way roller clutch which is permanently engaged but which only inhibits one direction of rotation. The second type is a multiple-disc clutch which is selectively engaged and which can control both directions of rotation. Table 1 will be discussed in more detail after the operation of these two types of clutches is explained.
Referring now also to FIGS. 2A, 2B, 3A and 3B, and in particular to FIGS. 2A and 2B, a schematic representation of a simple one-way roller clutch 50 is illustrated. The clutch 50 comprises a cam-cut drum 54 whose inner surface is machined with a series of angled grooves 56 into which rollers 52 are inserted. An outer surface, or outer race 55, of the drum 54 has teeth 57 which are splined to a member which is either permanently held stationary or selectively held stationary. The clutch 50 further comprises a hub 60 which has an inner surface, or inner race 58. Inner race 58 has teeth 62 which are splined to a member which is rotating (or, depending on the direction, which is trying to rotate).
The one-way roller clutch 50 unlocks when the hub 60 rotates clockwise, and locks when the hub 60 rotates counterclockwise. When the hub 60 tries to rotate in the clockwise direction, the rollers 52 are driven rightward and allow the hub 60 to rotate. When the hub 60 tries to rotate in the counterclockwise direction, the rollers 52 are driven leftward and wedge between the drum 54 and the hub 60, causing the clutch 50 to lock. Hence, the one-way roller clutch 50 holds the hub 60 in one direction, but not the other. Although the TH 350 has two different one-way roller clutches, the one that is of particular interest in the present invention is termed an intermediate one-way roller clutch.
Referring now in particular to FIG. 3A and 3B, a section 70 of a TH 350 having an intermediate one-way roller clutch 74 and an intermediate multiple disk clutch 71 is illustrated. The inner race of the intermediate one-way roller clutch 74 is splined to the front sun gear 26 via a high reverse clutch drum 77. The outer race is splined to the multiple disc clutch 71. The multiple disc clutch 71 is further splined to the transmission case 72, and selectively holds the intermediate one-way roller clutch 74 to the transmission case 72. The multiple disc clutch 71 will now be described in greater detail.
The intermediate multiple disc clutch 71 comprises a plurality of friction driven discs 76 and a plurality of steel drive discs 78. The friction discs 76 have teeth 92 on their inner edges which engage matching teeth on the outside of the intermediate one-way roller clutch 74. The friction discs 76 are alternated between steel discs 78 that have teeth 90 on their outer edges and engage matching teeth machined into the transmission case 72.
When the multiple disc clutch is not engaged, there are spaces between the friction discs 76 and the steel discs 78 and the two sets of discs 76, 78 may move freely with respect to each other. The multiple disc clutch 71 is applied when pressure of hydraulic input 86 is increased, causing a hydraulic annular piston 88 to force the plates 76, 78 together. In particular, two sets of plates 76, 78 are pressed between a pressure plate 80 (which is fixed and does not move relative to the transmission case 72) and a cushion spring 82 (which is acted on by the annular piston 88). The spaces between the friction discs 76 and the steel discs 78 are eliminated, and the two sets of discs 76, 78 grip one another thereby locking together and mechanically connecting the components engaged with their teeth.
From a system perspective, when the intermediate multiple disc clutch 71 is not engaged, and the plates 76, 78 are not locked together, the one-way roller clutch 74 and the front sun gear 26 are free to rotate in either direction. When the multiple disc clutch 71 is engaged, the drum of the roller clutch 74 is held and its rotation is prevented. When the drum of roller clutch 74 is held, the hub of roller clutch 74, and therefore the drum 77 and the front sun gear 26 are prevented from rotating counterclockwise. In short, when the multiple disc clutch 71 is not engaged, the front sun gear 26 is free to rotate in either direction; when the intermediate multiple disc clutch 71 is engaged, counterclockwise rotation of the front sun gear 26 is prevented.
As mentioned above, the multiple disc clutch is engaged when pressure at the hydraulic input 86 is increased. The pressure at the hydraulic input 86 is controlled by a hydraulic shift valve (not illustrated). The shift valve is acted on by throttle pressure and governor pressure which are opposing forces. Throttle pressure varies with engine output torque: throttle pressure is low when engine output torque is low, and is higher when engine output torque is high. For example, throttle pressure will be higher when the engine is accelerating or is operating under a heavy load. Governor pressure varies with speed: governor pressure is low when the output shaft of the transmission is rotating slowly, and is higher when the output shaft of the transmission is rotating quickly. When governor pressure exceeds throttle pressure (and a biasing spring pressure, if any), the shift valve "snaps" and allows mainline pressure to pass. Mainline pressure is high and will cause the multiple disc clutch 71 to engage when fed to the hydraulic input 86 of the multiple disc clutch 71.
In the TH 350, there are two one-way roller clutches. The operation of one of the one-way roller clutches, the intermediate one-way roller clutch, has already been described. The operation of the other one-way roller clutch, the low one-way roller clutch, is essentially the same in that it too prevents rotation in one direction but not the other. The main difference between the two clutches is that they hold different members of the transmission: the low one-way roller clutch is splined to the transmission case 72 and the rear carrier 44, whereas the intermediate one-way roller clutch is splined to the intermediate multiple disc clutch 71 and the front sun gear 26 (via the high reverse clutch drum 77). Since the operation of both roller clutches is essentially the same, the operation of each will hereinafter be described only at the system level.
Further, in the TH 350, there are three multiple disc clutches. Again, the operation of one of the multiple disc clutches, the intermediate multiple disc clutch 71, has already been described. The operation of the two other multiple disc clutches (the forward multiple disc clutch and the high reverse multiple disc clutch) is essentially the same in that they too use a series of friction discs which are alternated with steel drive discs. One main difference between the three multiple disc clutches is that they connect to different members of the transmission: the forward multiple disc clutch connects the input shaft to the front ring gear 14, the intermediate multiple disc clutch 71 connects the transmission case 72 to the intermediate one-way roller clutch 74, and the high-reverse clutch connects the front sun gear 26 to the input shaft. A second main difference between the three multiple disc clutches is that they engage at different throttle pressure/governor pressure combinations. Each of the multiple disc clutches is controlled by a separate shift valve, and although the shift valves are all controlled by the same throttle pressure and governor pressure, they use different biasing springs to vary the throttle pressure/governor pressure combination that will cause the valve to actuate. Since the operation of both roller clutches is otherwise essentially the same, the operation of each will hereinafter be described only at the system level.
Referring back to Table 1, the system level operation of the clutches 71, 74 can be better understood by comparing Table 1 to FIGS. 1A, 1B, 1C and 1D. In neutral, it is seen in Table 1 that none of the multiple disc clutches are engaged and, in particular, the forward multiple disc clutch is not engaged. The forward multiple disc clutch is the clutch that connects the input shaft of the transmission to the front ring gear 14 (which is the input member of the compound planetary gear system 10). Since the forward multiple disc clutch is not engaged, no members of the planetary gear system 10 are driven, and the output shaft of the transmission will not be driven.
In first speed, as illustrated in Table 1, the forward multiple disc clutch is engaged, and therefore the front ring gear 14 (i.e., the input member of the planetary gear set 10) is driven by the transmission input shaft. Note that the forward multiple disc clutch is always engaged in first, second, and third speed; the forward multiple disc clutch must be engaged in order for the front ring gear 14 to be driven. Further, the low one-way roller clutch holds the rear carrier 44 to the transmission case, preventing counterclockwise rotation of the rear carrier 44.
This information corresponds to that which is illustrated in FIGS. 1A and 1B. In FIG. 1A, the front ring gear 14 (which is itself driven by the transmission input shaft) drives the remaining members of the planetary gear set 10 except, as illustrated in FIG. 1B, the rear planet carrier 44 which is the only member held stationary. Hence, torque is transferred from the input shaft to the front ring gear 14, to the from carrier 24, to the front sun gear 26, to the rear sun gear 46, to the rear planet gears 40, to the rear ring gear 34, and to the output shaft. Note that if the rear planet carrier 44 was not held, it would rotate in a counterclockwise direction. However, as described above, the rear planet carrier 44 is held to the transmission case by the low one-way roller clutch and therefore does not rotate.
In second speed, as illustrated in Table 1, the forward multiple disc clutch is engaged and permits the front ring gear to be driven by the transmission input shaft. The intermediate multiple disc clutch 71 is also engaged and prevents counterclockwise rotation of the front sun gear 26. Rotation of the front sun gear 26 is prevented because the sun gear 26 is connected to the intermediate one-way roller clutch 74 (via the high reverse clutch drum), which is connected to the intermediate multiple disc clutch 71, which is splined to the transmission case 72.
This information corresponds to that which is illustrated in FIG. 1C. In FIG. 1C, the rotation of the front planetary gear set 12 while the transmission is in second speed is illustrated. (The rear planetary gear set 32 is not illustrated in second speed since, as described below, it does not rotate in second speed. ) The front ring gear 14 drives the front carrier assembly 24 (which is the output member), while the front sun gear 26 is held. Hence, torque is transferred directly from the input shaft to the front ring gear 14, to the front carrier 24, and to the output shaft. The front sun gear 26 (which is held by the intermediate one-way roller clutch) and the rear planetary gear set 32 do not rotate at all. The members of the rear planetary gear set 32 do not rotate since the front sun gear 26 (which is held) and the rear sun gear 46 share a common shaft 28. Further, since the rear planetary gear set 32 does not rotate, the low one-way roller clutch has no effect.
In third speed, as illustrated in Table 1, the high reverse multiple disc clutch connects the front sun gear 26 to the input shaft. As before, the front ring gear 14 is also connected to the input shaft (because the forward multiple disc clutch is engaged). Hence, since both the front ring gear 14 and the front sun gear 26 are connected to the input shaft, those two gears 14, 26 will rotate at the same speed. Since gears 14, 26 are rotating at the same speed, the planet carrier 24 also rotates at that speed. Since the planet carrier 24 is the output of the planetary gear set 10, it follows that there is a 1:1 gear ratio in third speed; in other words, third speed is a "direct drive" speed.
This information corresponds to that which is illustrated in FIG. 1D. In FIG. 1D, the rotation of the front planetary gear set 12 while the transmission is in second speed is illustrated. (The rear planetary gear set 32 is not illustrated in third speed since, as described, engine torque is not transferred through the rear gear set 32.) The front ring gear 14 and the front sun gear 26 are driven by the input shaft and rotate at the same speed. The gears 14, 26 drive the front carrier 24 which in turn drives the output shaft. Hence, torque is transferred from the input shaft, to the front ring gear 14 and front sun gear 26, to the front carrier 24, and to the output shaft.
The intermediate one-way roller clutch 74, and therefore the intermediate multiple disc clutch 71, have no effect in third speed. This is because the front sun gear 26 rotates clockwise in third speed, and the intermediate one-way roller clutch only prevents counterclockwise rotation. Further, note that the low one-way roller clutch has no effect. This is because the rear sun gear 46 rotates clockwise (since it shares a common shaft 28 with the front sun gear 26), and the low one-way roller clutch only prevents counterclockwise rotation.
As is apparent, transmissions are complex devices. The members of the planetary gear set cooperate in different ways at different speeds. Changing the operation of the transmission at one speed is likely to negatively impact the operation of the transmission at other speeds. Due to this complexity, it has been difficult until now to successfully convert a three speed transmission into a two speed transmission so that all of the remaining speeds function properly. A need therefore exists for a simple method of successfully converting a three speed transmission to a two speed transmission.